Nitride semiconductor luminous element and optical device including it

ABSTRACT

According to an aspect of the present invention, a nitride semiconductor light emitting device includes a light emitting layer having a quantum well structure with quantum well layers and barrier layers laminated alternately. The well layer is formed of a nitride semiconductor containing In, and the barrier layer is formed of a nitride semiconductor layer containing As, P or Sb. According to another aspect of the present invention, a nitride semiconductor light emitting device includes a light emitting layer having a quantum well structure with quantum well layers and barrier layers laminated alternately. The well layer is formed of GaN 1−x−y−   z As x P y Sb z  (0&lt;x+y+z≦0.3), and the barrier layer is formed of a nitride semiconductor containing In.

TECHNICAL FIELD

The present invention relates to nitride semiconductor light emittingdevices, and more particularly to a nitride semiconductor light emittingdevice improved in luminous efficiency.

BACKGROUND ART

Japanese Patent Laying-Open No. 10-270804 discloses a nitridesemiconductor light emitting device having a light emitting layerincluding GaNAs (or GaNP or GaNSb) quantum well layers and GaN barrierlayers. Further, Japanese Patent Laying-Open No. 11-204880 discloses anitride semiconductor light emitting device having a light emittinglayer including InGaNAs well layers and GaN barrier layers and emittinglight of a wavelength of more than 450 nm.

However, the inventors have found that, with these nitride semiconductorlight emitting devices of the prior art, it is difficult to improvesteepness in composition change at the interface (hereinafter, referredto as “interfacial steepness”) between the well and barrier layersincluded in the light emitting layer even if their growth conditions(including growth temperature) are controlled. Insufficient interfacialsteepness between the well and barrier layers causes increase ofhalf-width of emission peak, increase of color mottling, and degradationof luminous intensity (or gain reduction) in the light emitting device.Further, the fact that it is difficult to improve the interfacialsteepness between the well and barrier layers means that it is difficultto form a multiple quantum well structure including a plurality of welland barrier layers. Such problems are commonly seen in nitridesemiconductor light emitting devices having light emitting layersincluding GaNAs well layers/GaN barrier layers, GaNP well layers/GaNbarrier layers, GaNSb well layers/GaN barrier layers, InGaNAs welllayers/GaN barrier layers, InGaNP well layers/GaN barrier layers,InGaNSb well layers/GaN barrier layers and others.

DISCLOSURE OF THE INVENTION

In view of the above-described problems of the prior art, an object ofthe present invention is to improve interfacial steepness between welland barrier layers and hence to decrease threshold current density orimprove luminous intensity in a nitride semiconductor light emittingdevice.

According to the present invention, a nitride semiconductor lightemitting device formed on a substrate has a light emitting layerincluding a quantum well layer and a barrier layer in contact with thewell layer. The well layer is formed of a nitride semiconductorcontaining Ga, N and an element X, and the element X includes at leastone element selected from As, P and Sb. The well layer has an atomicfraction X/(N+X) of less than 30%. The barrier layer is formed of anitride semiconductor containing Ga, N and an element Y, and the elementY includes at least one element selected from As, P and Sb.

The well layer may further include In, in which case the atomic fractionX/(N+X) is preferably less than 20%.

The element X and the element Y may be the same element. A dose of theelement Y is preferably more than 1×10¹⁶/cm³.

The substrate is preferably a nitride semiconductor substrate or apseudo GaN substrate. An etch pit density of the substrate is preferablyless than 7×10⁷/cm².

The well layer preferably has a thickness of more than 0.4 nm and lessthan 20 nm, and the barrier layer preferably has a thickness of morethan 1 nm and less than 40 nm.

The light emitting layer preferably includes at least one impurityselected from Si, O, S, C, Ge, Zn, Cd and Mg, and a total dose of theimpurity is preferably more than 1×10¹⁶/cm³ and less than 1×10²⁰/cm³.

The number of the well layers is preferably at most 10, and preferablyat least 2.

An atomic fraction Y/(N+Y) is preferably equal to the atomic fractionN/(N+X). The atomic fraction Y/(N+Y) is preferably more than 2×10⁻⁵%.

Among the nitride semiconductor light emitting devices as describedabove, a nitride semiconductor laser device having a laser wavelength ofmore than 380 nm and less than 420 nm can preferably be used in anoptical pickup apparatus. Further, among the nitride semiconductor lightemitting devices as described above, a light emitting diode device or asuper-luminescent diode device having an emission wavelength of morethan 3.60 nm and less than 420 nm can preferably be used in a whitelight source apparatus. Still further, among the nitride semiconductorlight emitting devices as described above, a light emitting diode devicehaving an emission wavelength of more than 450 nm and less than 480 nmand a half-width of emission peak of less than 40 nm can preferably beused in a display apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view showing a structure of anitride semiconductor laser device according to an embodiment of thepresent invention.

FIG. 2 is a schematic cross sectional view showing a pseudo GaNsubstrate that can be used in formation of the nitride semiconductorlaser device according to the present invention.

FIG. 3A is a cross sectional view illustrating a process of forminganother pseudo GaN substrate, and FIG. 3B is a schematic cross sectionalview showing the pseudo GaN substrate formed through the process shownin FIG. 3A.

FIG. 4 is a schematic cross sectional view showing, a structure of anitride semiconductor laser device according to another embodiment ofthe present invention.

FIG. 5 is a schematic top plan view illustrating chip division of awafer with laser device structures formed.

FIG. 6 is a schematic cross sectional view showing a structure of anitride semiconductor light emitting diode device according to anotherembodiment of the present invention.

FIG. 7 is a schematic cross sectional view showing a structure of anitride semiconductor light emitting diode device according to yetanother embodiment.

FIG. 8 is a schematic block diagram of an optical disk apparatusincluding the nitride semiconductor laser device of the presentinvention.

FIG. 9 is a graph showing the relation between the number of well layersand the threshold current density in the nitride semiconductor laserdevices according to the present invention.

FIG. 10 is a graph showing the relation between the number of welllayers and the luminous intensity in the nitride semiconductor lightemitting diode devices according to the present invention.

FIG. 11A shows a bandgap structure of a light emitting layer including amultiple quantum well structure starting and ending with barrier layers,FIG. 11B shows a bandgap structure of a light emitting layer including amultiple quantum well structure starting and ending with well layers,and FIG. 11C shows a bandgap structure of a light emitting layerincluding a conventional multiple quantum well layer (Japanese PatentLaying-Open No. 10-270804).

FIG. 12 is a graph showing a SIMS result associated with As within aGaNAs barrier layer/GaNAs well layer/GaNAs barrier layer structure inthe nitride semiconductor light emitting device of the presentinvention.

FIG. 13 is a graph showing the relation between the dose of element Y ina GaNY barrier layer in contact with a GaNAs well layer and theinterfacial fluctuation.

FIG. 14 is a graph showing the relation between the dose of element Y ina GaNY barrier layer in contact With a GaNP well layer and theinterfacial fluctuation.

FIG. 15 is a graph showing the relation between the number of welllayers and the threshold current density in the nitride semiconductorlaser devices according to another embodiment of the present invention.

FIG. 16 is a graph showing the relation between the number of welllayers and the threshold current density in the nitride semiconductorlight emitting diode devices according to another embodiment of thepresent invention.

FIG. 17 is a graph showing a SIMS result associated with As in a GaNAsbarrier layer/InGaNAs well layer/GaNAs barrier layer structure of thepresent invention (atomic fraction of As in the GaNAs barrierlayer<atomic fraction of As in the InGaNAs well layer).

FIG. 18 is a graph showing a SIMS result associated with As in a GaNAsbarrier layer/InGaNAs well layer/GaNAs barrier layer structure of thepresent invention (atomic fraction of As in the GaNAs barrierlayer>atomic fraction of As in the InGaNAs well layer).

FIG. 19 is a graph showing a SIMS result associated with As in a GaNAsbarrier layer/InGaNAs well layer/GaNAs barrier layer structure of thepresent invention (atomic fraction of As in the GaNAs barrierlayer=atomic fraction of As in the InGaNAs well layer).

FIG. 20 is a graph showing SIMS results associated with As and P in aGaNP barrier layer/InGaNAs well layer/GaNP barrier layer structure ofthe present invention.

FIG. 21 is a graph showing the relation between the dose of element Y ina GaNY barrier layer in contact with an InGaNAs well layer and theinterfacial fluctuation.

FIG. 22 is a graph showing the relation between the dose of element Y ina GaNY barrier layer in contact with an InGaNP well layer and theinterfacial fluctuation.

BEST MODES FOR CARRYING OUT THE INVENTION

Firstly, description is given as to how the inventors found problems ofconventional nitride semiconductor light emitting devices and how theyhave reached the present invention. Embodiments of the present inventionwill be described thereafter.

To examine details of the GaNAs well layer/GaN barrier layer structureincluded in the light emitting layer of the nitride semiconductor lightemitting device disclosed in Japanese Patent Laying-Open No. 10-270804,the inventors carried out analysis of the same structure by SIMS(secondary ion mass spectroscopy). When the well layer and the barrierlayer were both formed at a temperature in a growth temperature range(600-800° C.) suitable for the GaNAs well layer, the As concentrationchanged steeply at the interface of GaNAs well layer/GaN barrier layer,while the steepness of the As concentration change was considerablyimpaired at the interface of GaN barrier layer/GaNAs well layer.

On the other hand, when the GaNAs well layer was formed at a temperaturein the growth temperature range (600-800° C.) suitable therefor and theGaN barrier layer was formed at a growth temperature (higher than 900°C.) suitable therefor, the As concentration changed steeply at theinterface of GaN barrier layer/GaNAs well layer, whereas the steepnessof the As concentration change was considerably impaired at theinterface of GaNAs well layer/GaN barrier layer, contrary to the abovecase.

It was found from the foregoing that, no matter how the growthconditions (growth temperatures) of the GaNAs well layer and the GaNbarrier layer included in the light emitting layer of the conventionalnitride semiconductor device were controlled, it would be difficult toimprove the steepness of As concentration change at the GaNAs welllayer/GaN barrier layer interface and the steepness of As concentrationchange at the GaN barrier layer/GaNAs well layer interface at the sametime. Poor interfacial steepness between the well and barrier layerscauses increase of the half-width of emission peak Beading to increaseof color mottling in the light emitting device) and decrease of luminousefficiency (leading to increase of threshold current density or decreaseof luminous intensity) attributable to the degraded interfacialcharacteristics. In addition, impairment of the interfacial steepnessbetween the well and barrier layers suggests difficulty in forming amultiple quantum well structure including a plurality of well andbarrier layers.

The inventors also carried out the SIMS analysis of the InGaNAs welllayer/GaN barrier layer structure included in the light emitting layerof the nitride semiconductor light emitting device disclosed in JapanesePatent Laying-Open No. 11-204880, and obtained the similar results as inthe case of the GaNAs well layer/GaN barrier layer structure.

The above-described problem of poor interfacial steepness between thewell and barrier layers occurs not only in the GaNAs well layer/GaNbarrier layer structure and the InGaNAs well layer/GaN barrier layerstructure, but also in GaNP well layer/GaN barrier layer structure,GaNSb well layer/GaN barrier layer structure, InGaNP well layer/GaNbarrier layer structure, and InGaNSb well layer/GaN barrier layerstructure.

First Embodiment

(Light Emitting Layer)

To solve the problem (of interfacial steepness) of the conventionallight emitting layer as described above, in a first embodiment of thepresent invention, a GaN barrier layer in contact with a GaNX well layer(containing at least one element selected from As, P and Sb as anelement X, and having an atomic fraction X/(N+X) of less than 30%)contains a third element Y. At least one element selected from As, P andSb can be contained as the element Y. Hereinafter, such a barrier layeris referred to as the GaNY barrier layer.

FIG. 12 shows a result of SIMS carried out in the light emitting layerincluding GaNAs well layers/GaNAs barrier layers in the presentembodiment. In FIG. 12, the well and barrier layers were formed at thesame growth temperature (800° C.). As seen from the drawing, interfacialsteepness between the well and barrier layers is improved by using theGaNAs barrier layer containing As as the element Y. This means that itis possible to form a multiple quantum well structure including aplurality of well and barrier layers.

Since the light emitting layer of the nitride semiconductor lightemitting device according to the present embodiment does not contain In,it does not cause the problems of color mottling and degradation inluminous intensity of the light emitting device attributable to phaseseparation of In.

The above-described effects of the present embodiment can be obtainednot only for the GaNAs well layer/GaNAs barrier layer structure, butalso for any GaNX well layer/GaNY barrier layer structure. The GaNX welllayer may be GaNP, GaNSb, GaNAsP, GaNAsPSb or the like, and the GaNYbarrier layer may be GaNAs, GaNP, GaNSb, GaNAsP, GaNAsPSb or the like.

In these mixed crystals, a ternary mixed crystal of GaNAs, GaNP or GaNSbis easy of controlling its composition ratio compared to a quaternarymixed crystal of GaNAsP or a quinary mixed crystal of GaNAsPSb, and thusmakes it possible to form a light emitting device having an intendedemission wavelength with good reproducibility.

The ternary mixed crystal of GaNP contains P having an atomic radius(Van der Waals radius or covalent bond radius) closest to that of Namong P, As and Sb. Thus, P atoms are more likely to be substituted withsome of N atoms in the mixed crystal, as compared to As or Sb atoms.Then, it is not likely that crystallinity of the GaN crystal is impairedby the addition of P. This means that crystallinity of the mixed crystalGaNP will not be degraded even if the composition ratio of P increases.Therefore, it is advantageous to use GaNP for the well layer of thelight emitting device in order to realize light emission in a widewavelength band from ultraviolet to red.

GaNSb contains Sb having the largest atomic radius among P, As and Sbcompared to N. This prevents highly volatile N atoms from escaping fromthe mixed crystal, which is preferable for improvement of crystallinity.

GaNAs contains As having an intermediate atomic radius among P, As andSb, and is preferable in view of having both characteristics of GaNP andGaNSb.

(Barrier Layer)

FIGS. 11A and 11B show bandgap structures applicable to the lightemitting layer of the nitride semiconductor light emitting deviceaccording to the present embodiment. FIG. 11A shows the bandgapstructure of the light emitting layer having a multiple quantum wellstructure starting with a barrier layer and ending with a barrier layer.FIG. 11B shows the bandgap structure of the light emitting layer havinga multiple quantum well structure starting with a well layer and endingwith a well layer. FIG. 11C shows the bandgap structure of the lightemitting layer according to Japanese Patent Laying-Open No. 10-270804.

Since the barrier layer of the present embodiment is formed of GaNY, thebandgap energy can be made smaller than that of the GaN light guidelayer (see FIGS. 11A and 11B). Accordingly, the multiple quantum welleffect due to sub-bands can readily be obtained compared to theconventional light emitting layer (FIG. 11C), the refractive index ofthe light emitting layer becomes greater than that of the light guidelayer, leading to improved light confinement efficiency, and the(unimodal) characteristic of the vertical transverse mode is alsoimproved.

(Dose of Element X in GaNX Well Layer)

The GaNX well layer of the present embodiment has an atomic fractionX/(N+X) (hereinafter, called “atomic fraction of element X”) of lessthan 30%, preferably less than 20%, and more preferably less than 10%,for the following reasons. If the atomic fraction of element X isgreater than 20%, phase separation (concentration separation) begins togradually occur and thus the atomic fraction of element X becomesdifferent in local regions within the well layer. Further, if the atomicfraction of element X exceeds 30%, the concentration separation proceedsto crystal system separation into a hexagonal system and a cubic system.Such crystal system separation considerably impairs the interfacialsteepness between the well and barrier layers. Further, if the ratio ofregions suffering the crystal system separation becomes more than about50% in the well layer, crystallinity of the well layer will also bedegraded considerably.

The lower limit of the atomic fraction of element X is at least 0.01%,and preferably at least 0.1%. This is because, if it is less than 0.01%,it is almost impossible to obtain the effect of adding element X in thewell layer (reduction of threshold current density or improvement ofluminous intensity). The atomic fraction of element X is preferably morethan 0.1% in order to surely have the effect of addition of element X inthe well layer.

(Dose of Element Y in GaNY Barrier Layer)

To find a preferable dose of element Y in the GaNY barrier layer, it wasexamined how the dose of element Y affect the interfacial steepnessbetween the well and barrier layers.

FIG. 13 shows measurement results of interfacial fluctuation in the casethat element Y was added to the GaN barrier layer within the GaN barrierlayer/GaNAs well layer structure. In this case, As, P or Sb was added aselement Y. A GaN substrate or a sapphire substrate was used as thesubstrate.

FIG. 14 shows, similarly to FIG. 13, measurement results of interfacialfluctuation in the case that element Y was added to the GaN barrierlayer within the GaN barrier layer/GaNP well layer structure.

Here, the interfacial fluctuation is represented as a depth (orthickness) from a point of the maximum secondary ion intensity in theSIMS measurement to another point of the minimum intensity or the otherway about (see FIG. 12). In FIGS. 13 and 14, the interfacial fluctuationis shown as an average value of the interfacial fluctuations at such awell layer/barrier layer interface and a barrier layer/well layerinterface as shown in FIG. 12. Further, black marks in the graphsrepresent interfacial fluctuations with respect to doses of element Y inthe light emitting device formed on a sapphire substrate (as an exampleof the substrate other than the nitride semiconductor substrate), andwhite marks represent interfacial fluctuations with respect to doses ofelement Y in the light emitting device formed on a GaN substrate (as anexample of the nitride semiconductor substrate).

As seen from FIGS. 13 and 14, the interfacial fluctuation can besuppressed if the total dose of element Y is more than 1×10¹⁶/cm³ (morethan 2×10⁻⁵% in terms of the atomic fraction of element Y), irrespectiveof the element species of As, P or Sb. The upper limit of the atomicfraction of element Y is preferably at most 15%. By the way, the energygap of the well layer needs to be smaller than that of the barrierlayer. Preferably, the bandgap energy of the barrier layer is made morethan 0.1 eV greater than that of the well layer. Good crystallinity canbe maintained when the atomic fraction of element Y is less than 15%.

As shown in FIGS. 13 and 14, the interfacial fluctuation in the lightemitting layer that was crystal grown using a GaN substrate was smallerthan in the case of using a sapphire substrate, the reason of which willbe described in detail in a later item (Substrate for Growing LightEmitting Device). The relation between the dose of element Y and theinterfacial fluctuation in the case using the pseudo GaN substrate wasapproximately the same as in the case of using the GaN substrate. Sincethe pseudo GaN substrate includes coexistent portions of high defectdensity and of low defect density, it is liable to degrade the yield ofthe light emitting devices. On the other hand, the pseudo GaN substratehas an advantage that one having a large area can be manufactured at lowcosts compared to the nitride semiconductor substrate. The effectsdescribed in conjunction with FIGS. 13 and 14 can be obtained with anywell layer of GaNX, not limited to GaNAs or GaNP.

(Thickness of Light Emitting Layer)

The GaNX well layer in the light emitting layer preferably has athickness of more than 0.4 nm and less than 20 nm. If the GaNX welllayer is thinner than 0.4 nm, the carrier confining level by the quantumwell effect becomes too high, possibly degrading luminous efficiency. Ifthe GaNX well layer is thicker than 20 nm, the interfacial steepnessbetween the well and barrier layers begins to degrade, though it dependson the atomic fraction of element X. This is presumably because phaseseparation due to element X slightly occurs even if the atomic fractionof element X in the GaNX well layer is less than 20%, and the regionssuffering such phase separation gradually increase as the thickness ofthe well layer increases. This leads to an irregular surface of the welllayer or even proceeds to crystal system separation.

The GaNY barrier layer in the light emitting layer has a thickness ofpreferably more than 1 nm and less than 40 nm, and more preferably morethan 1 nm and less than 20 nm. If the GaNY barrier layer is thinner than1 nm, it is difficult to fully confine the carriers within the welllayer. If the GaNY barrier layer is thicker than 40 nm, the interfacialsteepness between the well and barrier layers starts to degrade,presumably for the same reasons as in the case of the GaNX well layerdescribed above. The upper limit in thickness of the GaNY barrier layercan be about 20 nm greater than that of the GaNX well layer, because thedose of element Y is smaller than that of element X.

(Combination of GaNX Well Layer and GaNY Barrier Layer)

In the light emitting layer, element X of the GaNX well layer andelement Y of the GaNY barrier layer are preferably the same element.When element X and element Y are the same, it is unnecessary to changethe source material for the group V elements in depositing the welllayer and the barrier layer (eliminating a time lag required for thechange of source materials), which facilitates formation of the lightemitting layer. Further, when elements X and Y are the same, the welland barrier layers can be deposited approximately at the sametemperature. These advantages also contribute toward improvement of theinterfacial steepness between the well and barrier layers.

As examples of the combination of the well and barrier layers having thesame elements X and Y, it is possible to employ GaNAs well layer/GaNAsbarrier layer, GaNP well layer/GaNP barrier layer, GaNSb welllayer/GaNSb barrier layer, GaNAsP well layer/GaNAsP barrier layer, andGaNAsPSb well layer/GaNAsPSb barrier layer.

(Substrate for Growing Light Emitting Device)

The inventors have found that the interfacial fluctuation between theGaNX well layer and the GaNY barrier layer in the light emitting devicechanges depending on the substrate for growing the light emittingdevice. According to the findings of the inventors, As, P or Sb isliable to segregate near lattice defects in the grown crystal. As such,it is considered that proper selection of the substrate can reduce thedefect density in the grown crystal and improve the interfacialfluctuation between the well and barrier layers. The reason is becauseit is considered that the segregation of As, P or Sb near the defectsadversely affects the interfacial fluctuation.

Further, according to the findings of the inventors, the most preferablesubstrate is a GaN substrate (as an example of the nitride semiconductorsubstrate). A nitride semiconductor film grown on the GaN substrate hadan etch pit density of less than 5×10⁷/cm². This is smaller than theetch pit density (more than 4×10⁸/cm²) in the case of employing asapphire or SiC substrate (as an example of the substrate other than thenitride semiconductor substrate) conventionally used as a substrate fora nitride semiconductor light emitting device. Here, in measurement ofthe etch pit density, an epi-wafer (including the light emitting devicestructure) is immersed in an etchant (at a temperature of 250° C.) ofphosphoric acid to sulfuric acid=1:3 for 10 minutes, to form etch pitson the wafer surface. This etch pit density obtained by measuring thepit density on the epi-wafer surface may not exactly show the density ofdefects in the light emitting layer. However, the measurement of theetch pit density can indicate whether there are a large number ofdefects in the light emitting layer or not, since the density of defectsin the light emitting layer increases as the etch pit density increases.

The next favorable substrate as the nitride semiconductor substrate is apseudo GaN substrate. The method of forming the pseudo GaN substratewill be described in detail in the subsequent second embodiment. Thenitride semiconductor film grown on the pseudo GaN substrate had an etchpit density of less than 7×10⁷/cm² in a region of the lowest etch pitdensity, which is close to the etch pit density of the nitridesemiconductor film grown on the GaN substrate (as the nitridesemiconductor substrate). However, since the pseudo GaN substrate hascoexistent regions of low etch pit density (defect density) and highetch pit density, the yield of the light emitting devices is liable todecrease compared to the case of using the GaN substrate. On the otherhand, the pseudo GaN substrate is advantageous in that one having alarge area can be formed at low costs.

The light emitting layer grown on a GaN substrate is explained inconjunction with FIGS. 13 and 14 again. As seen from FIGS. 13 and 14,the light emitting layer grown on the GaN substrate includes lessinterfacial fluctuation than the light emitting layer grown on asapphire substrate.

It will be described in detail in the following item (Number of WellLayers) how the effect of suppressing interfacial fluctuation by usingthe GaN substrate contributes to the nitride semiconductor lightemitting device. To briefly set forth only the result, as seen fromFIGS. 9 and 10, it contributes to reduction of threshold current densityin the case of a nitride semiconductor laser device, and contributes toincrease of luminous intensity in the case of a light emitting diode.The relation between the dose of element Y and the interfacialfluctuation in the case of using the pseudo GaN substrate wasapproximately the same as in the case of using the GaN substrate inFIGS. 13 and 14.

(Number of Well Layers)

In the nitride semiconductor light emitting device according to thepresent embodiment, poor interfacial steepness of GaNAs well layer/GaNbarrier layer or the like can be improved. Such improvement of theinterfacial steepness between the well and barrier layers enablesformation of a desirable multiple quantum well structure including aplurality of well and barrier layers, and then it is expected to improvecharacteristics of the light emitting device including such a multiplequantum well structure.

Description is now given as to how the number of well layers and thekind of substrate affect lasing threshold current density in a nitridesemiconductor laser device including a multiple quantum well structure.FIG. 9 shows the relation between the number of well layers constitutingthe light emitting layer (multiple quantum well structure) and thelasing threshold current density. The light emitting layer used in FIG.9 includes, a GaN_(0.97)P_(0.03) well layer and a GaN_(0.99)P_(0.01)barrier layer. White circles and black circles in the figure representlasing threshold current densities in the case of using a sapphiresubstrate (as an example of the substrate other than the nitridesemiconductor substrate) and a GaN substrate (as an example of thenitride semiconductor substrate), respectively. The method of formingthe nitride semiconductor laser device on the GaN substrate is the sameas will be described in a later item (Method of Forming NitrideSemiconductor Laser). The method of forming the nitride semiconductorlaser device on the sapphire substrate is the same as will be describedlater in the third embodiment.

Referring to FIG. 9, continuous light emission at room temperature waspossible when the number of well layers was at most 10, irrespective ofthe kind of substrate. The well layers of at least 2 and at most 6 werepreferable to further reduce the lasing threshold current density. Ithas been found from the foregoing that a desirable multiple quantum wellstructure can be obtained by employing the barrier layer of the presentembodiment in the nitride semiconductor light emitting device. Further,it has been found that the semiconductor laser device using the GaNsubstrate has a lower threshold current density than the one using thesapphire substrate, as shown in FIG. 9. The relation between the numberof well layers and the threshold current density in the case of thelight emitting device formed on a pseudo GaN substrate was almost thesame as in the case of the light emitting device formed on the GaNsubstrate in FIG. 9.

Although the light emitting layer including GaN_(0.97)P_(0.03) welllayer/GaN_(0.99)P_(0.01) barrier layer has been explained in conjunctionwith FIG. 9, the relation between the number of well layers and thethreshold current density as in FIG. 9 can be obtained using any otherlight emitting layer as long as which satisfies the requirements of thepresent embodiment.

Now, description is given as to how the number of well layers and thekind of substrate affect luminous intensity in a nitride semiconductorlight emitting diode device including a multiple quantum well structure.FIG. 10 shows the relation between the number of well layers included inthe light emitting layer and the luminous intensity. The light emittinglayer used in FIG. 10 includes a GaN_(0.94)P_(0.06) well layer and aGaN_(0.995)P_(0.005) barrier layer. The luminous intensity in FIG. 10 isnormalized with the luminous intensity (broken line) of a conventionalnitride semiconductor light emitting device including a single quantumwell layer of GaN_(0.94)P_(0.06) . White circles and black circles inthe drawing represent the diode devices formed on a sapphire substrate(as an example of the substrate other than the nitride semiconductorsubstrate) and a GaN substrate (as an example of the nitridesemiconductor substrate), respectively. The methods of forming theselight emitting diode devices are the same as will be described below inthe fourth embodiment.

When the light emitting layer (GaN_(0.94)P_(0.06) well layer/GaN barrierlayer) in the conventional nitride semiconductor light emitting diodedevice was replaced with the light emitting layer (number of the welllayers was varied from 1 to 20) of the present embodiment, the maximumluminous intensity was about 1.4 times the reference luminous intensityfor normalization in FIG. 10. This means that the light emitting layerin the nitride semiconductor light emitting diode device of the presentembodiment is superior to the light emitting layer in the conventionallight emitting diode device.

Further, as seen from the luminous intensity in FIG. 10, the number ofwell layers in the nitride semiconductor light emitting diode device ispreferably at most 10, and more preferably at least 2 and at most 6,irrespective of the kind of substrate. Further, the luminous intensitybecomes higher by using the GaN substrate rather than the sapphiresubstrate. The relation between the number of well layers and theluminous intensity in the case of using a pseudo GaN substrate wasapproximately the same as in the case of using the GaN substrate shownin FIG. 10. The similar result as in FIG. 10 can also be obtained in asuper-luminescent diode device.

Although the light emitting layer including GaN_(0.94)P_(0.6) welllayer/GaN_(0.995)P_(0.005) barrier layer has been explained in FIG. 10,any other light emitting layer satisfying the requirements of thepresent embodiment can cause the similar relation between the number ofwell layers and the luminous intensity as shown in FIG. 10.

(Impurity in Light Emitting Layer)

Description is given about addition of impurity into the light emittinglayer in the nitride semiconductor light emitting device of the presentembodiment. According to photoluminescence (PL) measurement, PL luminousintensity increased to about 1.2-1.4 times when Si was added in thelight emitting layer (in both the barrier and well layers). This meansthat it is possible to improve the characteristics of the light emittingdevice by adding the impurity in the light emitting layer.

Since the light emitting layer of the present embodiment does notcontain In and then a local energy level due to In is not generated, theluminous intensity strongly depends on crystallinity of the well layer(the barrier layer in contact with the well layer also needs to havegood crystallinity to ensure good crystallinity of the well layer). Assuch, it is considered that addition of the impurity Si has served toimprove the crystallinity of the light emitting layer. In particular,the effect of improving the crystallinity of the light emitting layer byadding the impurity was remarkable in the case that the crystal growthwas performed, e.g., on a sapphire substrate other than the nitridesemiconductor substrate, in which case the defect density in the lightemitting layer was liable to increase (etch pit density: 4×10⁸/cm² ormore).

The similar effects can be obtained by adding at least one selected fromO, S, C, Ge, Zn, Cd and Mg, besides Si. All that is needed is that atotal dose of impurity is 1×10¹⁶/cm³ to 1×10²⁰/cm³. If the dose ofimpurity is less than 1×10¹⁶/cm³, the luminous intensity is notimproved. If it is greater than 1×10²⁰/cm³, the defect density due tothe impurity increases, leading to degradation of the luminousintensity.

(Light Emitting Layer and Emission Wavelength)

The emission wavelength from the light emitting layer of the nitridesemiconductor light emitting device of the present embodiment can bechanged by adjusting the atomic fraction of element X in the GaNX welllayer. For example, ultraviolet light with a wavelength in the vicinityof 380 mm can be obtained with x=0.005 in the case of GaN_(1−x)As_(x),y=0.01 in the case of GaN_(1−y)P_(y), and z=0.002 in the case ofGaN_(1−z)Sb_(z). Violet light with a wavelength in the vicinity of 410nm is obtained with x=0.02 in the case of GaN_(1−x)As_(x), y=0.03 in thecase of GaN_(1−y)P_(y), and z=0.01 in the case of GaN_(1−z)Sb_(z).Further, blue light with a wavelength in the vicinity of 470 nm isobtained with x=0.03 in the case of GaN_(1−x)As_(x), y=0.06 in the caseof GaN_(1−y)P_(y), and z=0.02 in the case of GaN_(1−z)Sb_(z).Furthermore, green light with a wavelength in the vicinity of 520 nm isobtained with x=0.05 in the case of GaN_(1−x)As_(x), y=0.08 in the caseof GaN_(1−y)P_(y), and z=0.03 in the case of GaN_(1−z)Sb_(z). Stillfurther, red light with a wavelength in the vicinity of 650 nm isobtained with x=0.07 in the case of GaN_(1−x)As_(x), y=0.12 in the caseof GaN_(1−y)P_(y), and z=0.04 in the case of GaN_(1−z)Sb_(z). Anemission wavelength almost as intended can be obtained by selecting aproper atomic fraction of the GaNX well layer taking into account therelation between the color and the atomic fraction as mentioned above.

(Light Emitting Layer and Half-Width of Emission Peak)

Description is given about the half-width of emission peak in the casethat the light emitting layer of the present embodiment is used to formsuch a light emitting diode of the fourth embodiment as will bedescribed later. Here, the half-width of emission peak means awavelength width at half the maximum luminous intensity in the emissionwavelength distribution profile of a light emitting diode operating atroom temperature.

In the nitride semiconductor light emitting device of the presentembodiment, interfacial steepness between the well and barrier layers isimproved compared to a conventional light emitting device, which leadsto reduction of the half-width of emission peak. Accordingly, it ispossible to form a light emitting device (light emitting diode) sharp inhue with less color mottling. More specifically, a conventional nitridesemiconductor light emitting device (including GaNAs well layers and GaNbarrier layers) emitting blue light in the wavelength range from about450 nm to about 480 nm had the half-width of emission peak of 60 nm, or0.35 eV in terms of energy width. By comparison, a nitride semiconductorlight emitting device (including GaNAs well layers and GaNAs barrierlayers) of the present embodiment similarly emitting blue light had thehalf-width of emission peak of 40 nm, or 0.25 eV in terms of energywidth.

Although the improvement in half-width of emission peak in the lightemitting device including the GaNAs well layers has been describedabove, the similar improvement can be achieved in a light emittingdevice including any other well layers, as long as the requirements ofthe present embodiment are met.

(Method of Forming Nitride Semiconductor Laser Device)

An example of the method of forming the nitride semiconductor laserdevice in FIG. 1 according to the present embodiment is now explained.

The nitride semiconductor laser device of FIG. 1 includes: a C (0001)plane n type GaN substrate 100; a GaN buffer layer 101; an n type GaNlayer 102; an n type In_(0.07)Ga_(0.93)N anti-crack layer 103; an n typeAl_(0.1)Ga_(0.9)N clad layer 104; an n type GaN light guide layer 105; alight emitting layer 106; a p type Al_(0.2)Ga_(0.8)N carrier block layer107; a p type GaN light guide layer 108; a p type Al_(0.2)Ga_(0.9)N cladlayer 109; a p type GaN contact layer 110; an n electrode 111; a pelectrode 112; and a SiO₂ dielectric film 113.

Firstly, n type GaN substrate 100 is set in a MOCVD (metallorganicchemical vapor deposition) system, and NH₃ (ammonia) as a sourcematerial for group V element and TMGa (trimethylgallium) or TEGa(tryethylgallium) as a source material for group III element are used togrow GaN buffer layer 101 to a thickness of 100 nm at a relatively lowsubstrate temperature of 550° C. Next, SiH₄ (silane) is added to thesource materials described above to grow n type GaN layer 102 (Siimpurity concentration: 1×10¹⁸/cm³) to a thickness of 3 μm at a growthtemperature of 1050° C. Thereafter, the substrate temperature is loweredto about 700-800° C., and TMIn (trimethylindium) as a source materialfor group III element is added to grow n type In_(0.07)Ga_(0.93)Nanti-crack layer 103 to a thickness of 40 nm. The substrate temperatureis raised again to 1050° C., and TMAl (trimethylaluminum) or TEAl(triethylaluminum) as a source material for group III element is used togrow n type Al_(0.1)Ga_(0.9)N clad layer 104 (Si impurity concentration:1×10¹⁸/cm³) to a thickness of 0.8 μm. N type GaN light guide layer 105(Si impurity concentration: 1×10¹⁸/cm³) is then grown to a thickness of0.1 μm. Thereafter, the substrate temperature is lowered to 800° C., andPH₃ or TBP (tertiary butyl phosphine) as a source material for P isadded to form light emitting layer (of multiple quantum well structure)106 including 4 nm thick GaN_(0.97)P_(0.03) well layers and 8 nm thickGaN_(0.99)P_(0.01) barrier layers in order of barrier layer/welllayer/barrier layer/well layer/barrier layer/well layer/barrier layer.That is, light emitting layer 106 has 3-cycle well layers. At this time,SiH₄ (Si impurity concentration: 1×10¹⁸/cm³) is added to both thebarrier and well layers. A growth break interval of at least one secondand at most 180 seconds may be provided between growth of the barrierlayer and growth of the well layer, or between growth of the well layerand growth of the barrier layer. This can improve flatness of therespective layers and also decrease the half-width of emission peak.

AsH₃ or TBAs (tertiary butyl arsine) may be used to add As in the lightemitting layer, or TMSb (trimethyl antimony) or TESb (triethyl antimony)may be used to add Sb in the light emitting layer. NH₃ as the sourcematerial of N may be replaced with dimethyl hydrazine in the formationof the light emitting layer.

Next, the substrate temperature is raised again to 1050° C. tosuccessively grow 20 nm thick p type Al_(0.2)Ga_(0.8)N carrier blocklayer 107, 0.1 μm thick p type GaN light guide layer 108, 0.5 μm thick ptype Al_(0.1)Ga_(0.9)N clad layer 109, and 0.1 μm thick p type GaNcontact layer 110. As the p-type impurity, Mg (EtCP₂Mg:bisethylcyclopentadienyl magnesium) was added at a concentration from5×10¹⁹/cm³ to 2×10²⁰/cm³. The p type impurity concentration in p typeGaN contact layer 110 is preferably increased as it approaches theinterface with p electrode 112. This can reduce the contact resistanceof the p electrode. Further, oxygen may be added by a minute amountduring growth of the p type layers, to remove residual hydrogen in the ptype layers that hinders activation of the p type impurity Mg.

After the growth of p type GaN contact layer 110, the entire gas in thereactor of the MOCVD system is replaced with nitrogen carrier gas andNH₉, and the substrate temperature is decreased at a cooling rate of 60°C. /min. Supply of NH₃ is stopped when the substrate temperature isdecreased to 800° C. The substrate is maintained at that temperature forfive minutes, and then cooled to room temperature. The substrate istemporarily held at a temperature preferably in a range from 650° C. to900° C., for a time period preferably in a range from 3 minutes to 10minutes. The cooling rate from the holding temperature is preferablymore than 30° C./min. The film thus grown was evaluated by Ramanspectroscopy, and it was found that the film already had p typecharacteristic (with Mg activated) immediately after its growth, evenwithout conventional annealing for p type characteristics carried out ina nitride semiconductor light emitting device. The contact resistance ofthe p electrode was also reduced. When the conventional annealing forgiving p type characteristic was additionally applied, the activationratio of Mg further improved favorably.

Low-temperature GaN buffer layer 101 of the present embodiment may be alow-temperature Al_(x)Ga_(1−x)N buffer layer (0≦x≦1), or the layeritself may be omitted. With a GaN substrate commercially available atthe present, however, it is preferable to insert the low-temperatureAl_(x)Ga_(1−x)N buffer layer (0≦x≦1) to improve the unfavorable surfacemorphology of the GaN substrate. Here, the low-temperature buffer layerrefers to a buffer layer formed at a relatively low growth temperatureof 450-600° C. The buffer layer formed at a growth temperature in thisrange becomes polycrystalline or amorphous.

In_(0.07)Ga_(0.93)N anti-crack layer 103 of the present embodiment mayhave the In composition ratio of other than 0.07, or the layer itselfmay be omitted. However, the InGaN anti-crack layer is preferablyinserted when lattice mismatch between the clad layer and the GaNsubstrate is large.

Although the light emitting layer described in conjunction with FIG. 1has the multiple quantum well structure starting and ending both withthe barrier layers (see FIG. 11A), it may have a structure starting andending both with the well layers (see FIG. 11B). The number of welllayers is not limited to 3 as described above. The threshold currentdensity is sufficiently low with at most 10 well layers, enablingcontinuous light emission at room temperature. In particular, the welllayers of at least 2 and at most 6 are preferable, ensuring the lowthreshold current density (see FIG. 9).

While Si (SiH₄) has been added in both the well and barrier layers at aconcentration of 1×10¹⁸/cm³ in the light emitting layer of the presentembodiment, it may be added to only the barrier layer or none of thelayers. This is because the defect density is not high in the film grownusing the nitride semiconductor substrate, and thus there is aprobability that the adverse effect of light absorption (gain loss) dueto the impurity will surpass the desirable effect of improvedcrystallinity due to the impurity in the light emitting layer. In thiscase, the dose of the impurity to be added in the light emitting layeris preferably on the order of 1×10¹⁶/cm³ to 1×10¹⁹/cm³, and any of O, S,C, Ge, Zn, Cd and Mg, besides Si, may be employed as the impurity.

P type Al_(0.2)G_(0.8)N carrier block layer 107 of the presentembodiment may have the Al composition ratio of other than 0.2, or thecarrier block layer itself may be omitted. The threshold currentdensity, however, was lowered with provision of the carrier block layer,presumably because of its function to confine the carriers in the lightemitting layer. The Al composition ratio of the carrier block layer ispreferably set high to enhance the carrier confining effect. When the Alcomposition ratio is set low in the range guaranteeing the carrierconfinement, mobility of the carriers in the carrier block layerincreases, leading to favorable reduction of electric resistance.Further, since carrier block layer 107 contains Al, it can preventescape of elements X and Y from the light emitting layer.

Although Al_(0.1)Ga_(0.9)N crystals have been employed for the p typedad layer and the n type clad layer in the present embodiment, the Alcomposition ratio may be other than 0.1. If the Al composition ratio inthe clad layers is increased, the differences in energy gap andrefractive index compared with the light emitting layer increase, sothat carriers and light can be confined in the light emitting layerefficiently, which leads to reduction of the lasing threshold currentdensity. Further, if the Al composition ratio is lowered in the rangeensuring confinement of carriers and light, mobility of the carriers inthe clad layers will increase, so that operating voltage of the lightemitting device can be reduced.

The AlGaN clad layer preferably has a thickness of 0.7-1.5 μm. Thisensures a unimodal vertical transverse mode and increases the lightconfining effect, and further enables improvement in opticalcharacteristics of the laser and reduction of the threshold currentdensity.

The clad layer is not limited to the ternary system mixed crystal ofAlGaN, but it may be a quaternary system mixed crystal of AlInGaN,AlGaNP, or AlGaNAs. Further, the p type clad layer may have asuper-lattice structure formed of p type AlGaN layer and p type GaNlayer or a super-lattice structure formed of p type AlGaN layer and ptype InGaN layer for the purpose of reducing its electric resistance.

Although the C {0001} plane has been explained as the main surface ofthe GaN substrate in conjunction with FIG. 1, A {11-20} plane, R {1-102}plane, M {1-100} plane or {1-101} plane may also be employed as the mainsurface of the substrate. A substrate main surface having an off-angleof at most two degrees from any of these plane orientations can causegood surface morphology.

Although the GaN substrate has been used in FIG. 1, a nitridesemiconductor substrate other than the GaN substrate may also beemployed. In the case of a nitride semiconductor laser, it is preferablethat a layer having a lower refractive index than that of a clad layeris provided in contact with the outside of the clad layer to obtain aunimodal vertical transverse mode. As such, an AlGaN substrate may beused preferably.

Although the crystal growth using the MOCVD system has been explained inconjunction with FIG. 1, molecular beam epitaxy (MBE) or hydride vaporphase epitaxy (HVPE) may be used for the crystal growth.

The epi-wafer including the plurality of nitride semiconductor layersstacked as described above is taken out of the MOCVD system andprocessed to obtain laser devices.

Hf and Au are deposited in this order on the back side of n type GaNsubstrate 100, to form n electrode 111. Ti/Al, Ti/Mo, Hf/Al or the likemay also be used as the materials for the n electrode. Hf is preferablyused to decrease the contact resistance of the n electrode.

The p electrode portion is etched in a stripe manner along a <1-100>direction of the nitride semiconductor crystal, to form a ridge stripeportion as shown in FIG. 1. The ridge stripe portion is formed to have awidth of 2 in. Thereafter, SiO₂ dielectric film 113 is formed byevaporation, and an upper surface of p type GaN contact layer 110 isexposed. On the exposed surface of the contact layer, Pd, Mo and Au arevapor-deposited in this order to form p electrode 112. Pd/Pt/Au, Pd/Auor Ni/Au may also be used as the materials for the p electrode.

Thereafter, Fabri-Perot resonators of 500 μm each in length are formedutilizing the cleavage plane of the GaN substrate. In general, theresonator length is preferably in a range of from 300 μm to 1000 μm. Themirror end surfaces of each resonator are formed parallel to the M plane({1-100} plane) of the GaN substrate (see FIG. 5). Cleavage forformation of the mirror end surfaces and chip division into laserdevices are carried out with a scriber on the substrate side along thebroken lines shown in FIG. 5. The cleavage for forming the mirror endsurfaces is done with the scriber, by scratching not the entire wafersurface but only portions of the wafer, e.g., only the both ends of thewafer. This prevents degradation of steepness of the end surfaces andalso prevents shavings due to the scribing from attaching to theepi-surface, thereby increasing the yield of the laser devices. As thetype of the laser resonator, DFB (distributed feedback) type or DBR(distributed bragg reflector) type commonly known may also be employedbesides the Fabri-Perot type.

After formation of the mirror end surfaces of the Fabri-Perot resonator,dielectric films of SiO₂ and TiO₂ are alternately formed on one of themirror end surfaces by evaporation, to make a dielectric multilayerreflection film having a reflectance of 70%. Alternatively, SiO₂/Al₂O₃may be used as the materials for the dielectric multilayer reflectionfilm.

Although n electrode 111 has been formed on the back side of n type GaNsubstrate 100, alternatively a portion of n type GaN layer 102 may beexposed from the front side of the epi-wafer by dry etching, to form then electrode on the exposed portion (see FIG. 4).

The method of packaging the nitride semiconductor laser chip is nowexplained. In the case of using the nitride semiconductor laser deviceas a high output laser device (of at least 30 mW), the laser chip isconnected to a package body using an In soldering material, preferablyjunction-up, or more preferably junction-down. Alternatively, thenitride semiconductor laser chip may be connected via a sub-mount of Si,AIN, diamond, Mo, CuW, BN, Au, Cu or Fe, instead of being directlyattached to the package body or a heat sink portion. The nitridesemiconductor laser device of the present embodiment is formed asdescribed above.

Second Embodiment

The second embodiment differs from the first embodiment of FIG. 1 onlyin that GaN substrate 100 is replaced with a pseudo GaN substrate 200 ofFIG. 2 or a pseudo GaN substrate 200 a of FIG. 3B, and that the nelectrode is formed on the front side of the substrate as shown in FIG.4.

The pseudo GaN substrate 200 of FIG. 2 includes a seed substrate 201, alow-temperature buffer layer 202, an n type GaN film 203, a growthinhibiting film 204, and an n type GaN thick film 205. Seed substrate201 is used as a base material for growing n type GaN thick film 205.The growth inhibiting film refers to a film restricting crystal growthof the nitride semiconductor film thereon. The pseudo GaN substratedescribed here refers not only to the one having a structure shown inFIG. 2, but also to any other one including at least a seed substrateand a growth inhibiting film.

The pseudo GaN substrate 200 a of FIG. 3B includes a seed substrate 201,a low-temperature buffer layer 202, a first n type GaN film 203 a, and asecond n type GaN film 203 b. In forming pseudo GaN substrate 200 a, asshown in FIG. 3A, first n type GaN film 203 a is laminated on seedsubstrate 201, and then the surface of the GaN film is shaped to havegrooves by dry etching or wet etching. Thereafter, second n type GaNfilm 203 b is laminated, to complete pseudo GaN substrate 200 a (seeFIG. 3B). Although the grooves have been formed only to a depth in themiddle of first n type GaN film 203 a in FIG. 3A, they may be formed toreach into low-temperature buffer layer 202 or into seed substrate 201.

When a nitride, semiconductor film was grown using pseudo GaN substrate200 or 200 a formed as described above, its defect density (etch pitdensity: less than about 7×10⁷cm²) was lower than in the case of using asapphire or SiC substrate (etch pit density: more than about 4×10⁸/cm²).In the case of FIG. 2,the defect density was high in regions along aline 206 defining the center in width of growth inhibiting film 204 andalong a line 207 defining the center in width of the portion unprovidedwith the growth inhibiting film. In the case of FIG. 3B, the defectdensity was high in regions along a line 208 defining the center of agroove and along a line 209 defining the center in width of the portion(hill) unprovided with the groove. That is, the defect density is low ina region between lines 206 and 207 in FIG. 2 and in a region betweenlines 208 and 209 in FIG. 3B. Accordingly, a light emitting device canbe preferably formed in such low defect density regions on the pseudoGaN substrate.

As specific examples of seed substrate 201, it is possible to useC-plane sapphire, M-plane sapphire, A-plane sapphire, R-plane sapphire,GaAs, ZnO, MgO, spinel, Ge, Si, GaN, 6H-SiC, 4H-SiC and 3C-SiC.

As the material for growth inhibiting film 204, it is possible to usedielectric materials such as SiO₂, SiN., TiO₂ and Al₂O₃, and metal filmssuch as a tungsten film. Further, a hollow portion may be formed ongrowth inhibiting film 204 in FIG. 2. Provision of the hollow portion inn type GaN thick film 205 alleviates crystal strain above the hollowportion, and can accordingly contribute to improvement in luminousefficiency of the light emitting device.

When a conductive SiC or Si substrate is used as the seed substrate, then electrode may be formed on the back side of the substrate, as shown inFIG. 1. In such a case, however, low-temperature buffer layer 202 shouldbe replaced with a high-temperature buffer layer. Here, thehigh-temperature buffer layer refers to a buffer layer formed at agrowth temperature of at least 900° C. The high-temperature buffer layerneeds to contain at least Al; otherwise, it is impossible to form anitride semiconductor film of good crystallinity over the SiC or Sisubstrate. The most preferable material for the high-temperature bufferlayer is InAIN.

Preferably, the main surface orientation of the seed substrate (in thecase of hexagonal system) is C {0001} plane, A { 1-20} plane, R {1-102}plane, M {1-100} plane or {1-101} plane. Any main surface of thesubstrate having an off-angle within two degrees from any of these planeorientations ensures good surface morphology.

A nitride semiconductor laser device using the pseudo GaN substrate isnow explained with reference to FIG. 4. This nitride semiconductor laserdevice includes a substrate 300, a low-temperature GaN buffer layer 101,an n type GaN layer 102, an n type In_(0.07)Ga_(0.93)N anti-crack layer103, an n type Al_(0.1)Ga_(0.9)N clad layer 104, an n type GaN lightguide layer 105, a light emitting layer 106, a p type Al_(0.2)Ga_(0.9)Ncarrier block layer 107, a p type GaN light guide layer 108, a p typeAl_(0.1)Ga_(0.9)N clad layer 109, a p type GaN contact layer 110, an nelectrode 111, a p electrode 112, and a SiO₂ dielectric film 113.Substrate 300 is a pseudo GaN substrate. The method of forming thisnitride semiconductor laser device is similar to that of the firstembodiment, though such packaging as will be described below in thethird embodiment is preferable in the case of using the seed substrate,e.g., of sapphire having poor heat conductivity.

The nitride semiconductor laser device of the present embodiment has aridge stripe portion (see FIG. 1 or 3) that is formed in a regionavoiding lines 206 and 207 in FIG. 2 or lines 208 and 209 in FIG. 3.

Low-temperature GaN buffer layer 101 of the present embodiment may bereplaced with a low-temperature Al_(x)Ga_(1−x)N buffer layer (0≦x≦1), orthe low-temperature buffer layer itself may be omitted. If the pseudoGaN substrate has poor surface morphology, however, it is preferable toinsert the low-temperature Al_(x)Ga_(1−x)N buffer layer (0≦x≦1) toimprove the surface morphology.

Pseudo GaN substrate 300 may have its seed substrate 201 removed by apolishing machine. Alternatively, substrate 300 may have low-temperaturebuffer layer 201 and all the layers thereunder removed by a polishingmachine. Further, substrate 300 may have growth inhibiting film 204 andall the layers thereunder removed by a polishing machine. In the casethat seed substrate 201 is removed, n electrode 111 may be formed on thesurface from which the seed substrate has been removed. Still further,seed substrate 201 may be removed after the nitride semiconductor laserdevice is formed, on pseudo GaN substrate 300.

Third Embodiment

The third embodiment differs from the embodiment of FIG. 1 only in thatthe nitride semiconductor laser device is formed on a substrate otherthan the nitride semiconductor substrate with a nitride semiconductorbuffer layer interposed therebetween, and that the n electrode is formedon one side of the substrate as shown in FIG. 4.

Referring to FIG. 4, the nitride semiconductor laser device of thepresent embodiment includes a substrate 300, a low-temperature GaNbuffer layer 101 (thickness: 25 nm), an n type GaN layer 102, an n typeIn_(0.07)Ga_(0.93)N anti-crack layer 103, an n type Al_(0.1)Ga_(0.9)Nclad layer 104, an n type GaN light guide layer 105, a light emittinglayer 106, a p type Al_(0.2)Ga_(0.8)N carrier block layer 107, a p typeGaN light guide layer 108, a p type Al_(0.1)Ga_(0.9)N clad layer 109, ap type GaN contact layer 110, an n electrode 111, a p electrode 112, anda SiO₂ dielectric film 113. Substrate 300 may be, e.g., a C (0001) planesapphire substrate.

The method of forming this nitride semiconductor laser device is similaras in the first embodiment. In packaging, this laser device ispreferably connected with being junction-down to a package body, usingan In soldering material or the like. Alternatively, the laser devicemay be connected via a sub-mount of Si, AIN, diamond, Mo, CuW, BN, Au,Cu or Fe, instead of being directly attached to the package body or aheat sink portion. In the case that substrate 300 is formed of amaterial such as SiC or Si having high heat conductivity, the laserdevice may also be mounted junction-up.

Although the sapphire substrate has been employed for substrate 300 inthe present embodiment, 6H—SiC, 4H—SiC, 3C—SiC, Si or spinel (MgAl₂O₄)may be employed for the substrate. Since the SiC substrate and the Sisubstrate are conductive, the n electrode may be formed on the back sideof the substrate, as shown in FIG. 1. Further, the buffer layer forgrowing a nitride semiconductor film of good crystallinity over the SiCor Si substrate is such a high-temperature buffer layer as in the secondembodiment.

Although the C {10001} plane substrate has been explained in the presentembodiment, the main surface orientation of the substrate may also be A{11-20}plane, R {1-102} plane, M {1-100} plane or {1-101} Further, anymain surface of the substrate having an off-angle within two degreesfrom any of these plane orientations enables good surface morphology.

Fourth Embodiment

In the fourth embodiment, the case of applying the present invention toa nitride semiconductor light emitting diode device is explained. FIG. 6shows a cross section of the nitride semiconductor light emitting diodedevice.

This nitride semiconductor light emitting diode device includes an ntype GaN substrate 600 having a C (0001) plane, a low-temperature GaNbuffer layer 601 (thickness: 100 nm), an n type GaN layer 602(thickness: 3 μm, Si impurity concentration: 1×10¹⁸/cm³), a lightemitting layer 603 (including, e.g., five cycles of GaN_(0.97)As_(0.03)well layers each having a thickness of 3 nm and GaN_(0.99)As_(0.01)barrier layers each having a thickness of 6 nm), a p typeAl_(0.1)Ga_(0.9)N carrier block layer 604 (thickness: 20 nm, Mg impurityconcentration: 6×10¹⁹/cm³), a p type GaN contact layer 605 (thickness:0.1 μm, Mg impurity concentration: 1×10²⁰/cm³), a transparent electrode606, a p electrode 607, and an n electrode 608.

N electrode 608 of the present embodiment is formed by laminating metallayers of Hf and Au in this order on the back side of n type GaNsubstrate 100. Ti/Al, Ti/Mo or Hf/Al may also be used as the materialsof the n electrode. Use of Hf in the n electrode is particularlypreferable, since it reduces the contact resistance of the n electrode.Although n electrode 608 of the present embodiment has been formed onthe back side of n type GaN substrate 600, a portion of n type GaN layer602 may be exposed on the p electrode side of the epi-wafer by dryetching, as shown in FIG. 7, and n electrode 608 may be formed on theexposed portion.

On the p electrode side, a Pd film as transparent electrode 606 isformed by evaporation to a thickness of 7 nm, and an Au film as pelectrode 607 is also formed by evaporation. Alternatively, thetransparent electrode may be formed of Ni, Pd/Mo, Pd/Pt, Pd/Au, orNi/Au.

Thereafter, chip division is carried out, using a scriber on the backside of n type GaN substrate 600 (opposite to the side on whichtransparent electrode 606 is formed by evaporation). Scribing is carriedout in such a direction that at least one side of the chip includes thecleavage plane of the nitride semiconductor substrate. This preventsabnormality in chip shape due to chipping or cracking, and improves theyield of chips per wafer.

The nitride semiconductor substrate (GaN substrate 600) may be replacedwith the pseudo GaN substrate explained in the second embodiment. Thenitride semiconductor light emitting diode device in the case of usingthe pseudo GaN substrate has approximately the same characteristics asin the case of using the nitride semiconductor substrate (see FIG. 10).However, the yield of the light emitting devices is liable to decreasein the case of using the pseudo GaN substrate compared to the case ofusing the nitride semiconductor substrate, since the pseudo GaNsubstrate includes coexistent regions of low etch pit density (defectdensity) and of high etch pit density. On the other hand, the pseudo GaNsubstrate is advantageous in that it can be formed to have a large areaat low costs, compared to the case of the nitride semiconductorsubstrate. When the pseudo GaN substrate includes an insulative seedsubstrate, the p electrode and the n electrode may be formed on the sameside of the substrate, as shown in FIG. 7.

The nitride semiconductor light emitting diode device may be formed on asubstrate other than the nitride semiconductor substrate, with a nitridesemiconductor buffer layer interposed therebetween. A specific exampleof such a device is explained with reference to FIG. 7.

The nitride semiconductor light emitting diode device of FIG. 7 includesa substrate 300, a low-temperature GaN buffer layer 601 (thickness: 25nm), an n type GaN layer 602 (thickness: 3 ;μm, Si impurityconcentration: 1×10¹⁸/cm³), a light emitting layer 603 (e.g., five-cycleGaN_(0.94)P_(0.06) well layers/GaN_(0.99)P_(0.01) barrier layers), a ptype Al_(0.1)Ga_(0.9)N carrier block layer 604 (thickness: 20 nm, Mgimpurity concentration: 6×10¹⁹/cm³), a p type GaN contact layer 605(thickness: 0.1 μm, Mg impurity concentration: 1×10²⁰/cm³), atransparent electrode 606, a p, electrode 607, an n electrode 608, and adielectric film 609. Here, a sapphire substrate is used as the substrate300. In the case that substrate 300 is a conductive SiC or Si substrate,the n electrode and the p electrode may of course be formed on differentsides of the substrate, as shown in FIG. 6. In this case, the bufferlayer for enabling growth of a nitride semiconductor film of goodcrystallinity over the SiC or Si substrate is such a high-temperaturebuffer layer as in the second embodiment.

Fifth Embodiment

The fifth embodiment is similar to the above-described embodimentsexcept that the present invention is applied to a nitride semiconductorsuper-luminescent diode device. As to the luminous intensity of thislight emitting device, approximately the same results as in the nitridesemiconductor light emitting diode device can be obtained (see FIG. 10).

Sixth Embodiment

(Light Emitting Layer)

In the light emitting layer of the sixth embodiment, a Ga NY barrierlayer is provided in contact with an InGaNX well layer (of which atomicfraction of element X is more than 20%). That is, the sixth embodimentdiffers from the first embodiment in that the GaNX well layer isreplaced with the InGaNX well layer. SIMS results of the InGaNAs welllayer/GaNAs barrier layer of the present embodiment are shown in FIGS.17-19. In these figures, the well and barrier layers were formed at thesame growth temperature (800° C.).

Here, FIG. 17 shows the SIMS result in the case that the atomic fractionof As in the GaNAs barrier layer is smaller than that of the InGaNAswell layer. FIG. 18 shows the SIMS result when the atomic fraction of Asin the GaNAs barrier layer is greater than that of the InGaNAs welllayer. FIG. 19 shows the SIMS result when the GaNAs barrier layer andthe InGaNAs well layer have the same. As atomic fraction.

It is understood from these figures that using the barrier layer of thepresent embodiment can improve the interfacial steepness associated withAs. In the case of FIG. 19, it is natural that the secondary ionintensity associated with As is approximately uniform at the interface,since the barrier and well layers are equal in As atomic fraction. Theseresults indicate that it is possible to form a multiple quantum wellstructure consisting of a plurality of well and barrier layers.

Since the barrier layer of the present embodiment does not contain In,segregation due to In is not generated. The barrier layer not containingIn suppresses In segregation in the InGaNX well layer, thereby improvingthe interfacial steepness of the light emitting layer, for the followingreasons. In is highly liable to segregate (coagulate), and readilycauses separation Chase separation or concentration separation) of thenitride semiconductor crystal into regions high and low in atomicfraction of In (defined as In/(In+Ga)). In particular, the region withhigh In atomic fraction is liable to emit no light, thereby degradingluminous efficiency. The inventors have found that probability ofoccurrence of such phase separation due to In is in proportion to thetotal dose of In within the entire light emitting layer (well andbarrier layers) and to the total volume of the layers containing In.Thus, it is preferable to minimize the In dose in the light emittinglayer, and especially preferable to completely eliminate In from thebarrier layer as in the present embodiment.

The effect of the present embodiment is obtained not only in the case ofthe light emitting layer including InGaNAs well layer/GaNAs barrierlayer, but also in the case of a light emitting layer including InGaNXwell layer/GaNY barrier layer. As specific examples of the InGaNX welllayer/GaNY barrier layer, there are InGaNAs well layer/GaNP barrierlayer, InGaNAs well layer/GaNSb barrier layer, InGaNAs well layer/GaNAsPbarrier layer, InGaNP well layer/GaNP barrier layer, InGaNP welllayer/GaNAs barrier layer, InGaNP well layer/GaNSb barrier layer, InGaNPwell layer/GaNAsP barrier layer, InGaNSb well layer/GaNSb barrier layer,InGaNSb well layer/GaNAs barrier layer, InGaNSb well layer/GaNP barrierlayer, InGaNSb well layer/GaNAsP barrier layer, InGaNAsP welllayer/GaNAsP barrier layer, InGaNAsP well layer/GaNAs barrier layer, andInGaNAsP well layer/GaNP barrier layer.

SIMS results in the case of the light emitting layer including InGaNAswell layer/GaNP barrier layer are shown in FIG. 20,by way of example. Itshows that both As in the InGaNAs well layer and P in the GANP barrierlayer exhibit steep concentration changes at the interface. Theinterfacial steepness as in FIG. 20 can be obtained when the lightemitting layer includes InGaNP well layer/GaNAs barrier layer.

The ternary system mixed crystal of GaNAs, GaNP or GaNSb in the GaNYbarrier layers can be formed with an intended atomic fraction of elementY of the barrier layer with good reproducibility, because the atomicfraction of element Y is readily controllable compared to the quaternarysystem mixed crystal of GaNAsP. The ternary system mixed crystal alsohas the favorable characteristics as described in the first embodiment.

Similarly, the quaternary system mixed crystal of InGaNAs, InGaNP orInGaNSb in the InGaNX well layers can make it possible to obtain anintended emission wavelength with good reproducibility, because theatomic fraction of element X is readily controllable compared to thequinary system mixed crystal of InGaNAsP.

Further, in the quaternary system mixed crystals, InGaNP contains Phaving an atomic radius closest to that of N among P, As and Sb, so thatpart of N atoms in the mixed crystal is more readily substituted by Patoms compared with As or Sb atoms. Thus, crystallinity of InGaN isunlikely to be impaired by addition of P. That is, even if the atomicfraction of P within InGaNP increases, the crystallinity will not bedegraded. Therefore, use of InGaNP for the well layer is advantageous inrealizing a wide emission wavelength band from ultraviolet to red. Onthe other hand, InGaNSb is preferable because it improves crystallinityof the well layer for the same reason as GaNSb described in the firstembodiment. InGaNAm is also preferable for the same reason as GaNAsdescribed in the first embodiment.

(Combination of InGaNX Well Layer and GaNY Barrier Layer)

In the light emitting layer of the sixth embodiment, the similaradvantages as in the first embodiment can be obtained when element X inthe InGaNX well layer and element Y in the GaNX barrier layer are thesame element. In addition, the method of forming the light emittingdevice is further facilitated when elements X and Y are approximatelyequal in atomic fraction, in which case the interfacial steepness in thelight emitting layer is unnecessary, as shown in FIG. 19.

As specific examples of combination of InGaNX well layer and GaNYbarrier layer, there are InGaNAs well layer/GaNAs barrier layer, InGaNPwell layer/GaNP barrier layer, InGaNSb well layer/GaNSb barrier layer,and InGaNAsP well layer/GaNAsP barrier layer.

(GaNY Barrier Layer)

The bandgap structures as shown in FIGS. 11A and 11B in the firstembodiment can similarly be applied to the GaNY barrier layer of thesixth embodiment.

(Dose of Element Y in GaNY Barrier Layer)

FIG. 21 shows measurement results regarding the influence of the dose ofelement Y on the interfacial fluctuation in the GaNY barrierlayer/InGaNAs well layer structure. Here, as specific examples ofelement Y, there are As, P and Sb. Similarly, FIG. 22 shows measurementresults regarding the influence of the dose of element Y on theinterfacial fluctuation in the GaNY barrier layer/InGaNP well layerstructure. Here, the interfacial fluctuation is represented as a depth(thickness) from a point of the maximum secondary ion intensity obtainedin the SIMS measurement to another point of the minimum intensity or theother way about (see FIG. 17), similarly as in the first embodiment. InFIGS. 21 and 22, each of the interfacial fluctuations is indicated by avalue averaged for such both interfaces as shown in FIG. 17. Black andwhite marks in FIGS. 21 and 22 represent light emitting layers formedusing the sapphire substrate and the GaN substrate, respectively.

According to FIGS. 21 and 22, the interfacial fluctuation can besuppressed when a total dose of element Y is more than 1×10¹⁶/cm³ (morethan 2×10⁻⁵% in terms of atomic fraction), irrespective of As, P or Sb,similarly as in the first embodiment. Other effects of addition ofelement Y in the sixth embodiment are also the same as those in thefirst embodiment.

(Dose of Element X in InGaNX Well Layer)

The InGaNX well layer has an atomic fraction of element X of less than20%, preferably less than 15%, and more preferably less than 10%. If theatomic fraction of element X is greater than 15%, phase separationbegins to occur where the atomic fraction of element X becomes differentin local regions within the well layer, though it depends on the dose ofIn. When the atomic fraction of element X exceeds 20%, the phaseseparation proceeds further to cause crystal system separation where ahexagonal system and a cubic system coexist. Such crystal systemseparation leads to disadvantages as described in the first embodiment.The lower limit of the atomic fraction of element X is at least 0.01%,and preferably at least 0.1%, for the same reasons as in the firstembodiment.

(Atomic Fraction of In within InGaNX Well Layer)

In contained in the InGaNX well layer is a preferable element because itcan localize carriers to improve luminous efficiency. When the atomicfraction of In increases, however, phase separation may occur due to In,possibly degrading the luminous efficiency.

The atomic fraction of In within the InGaNX well layer can be adjustedin view of the atomic fraction of element X in accordance with anintended emission wavelength (or lasing wavelength) (see, e.g., Table 1or 2). When the intended emission wavelength is shorter than 470 nm, theatomic fraction of In is preferably from 0.1% to 20%, and morepreferably from 0.1% to 10%. If the atomic fraction of In is less than0.1%, improvement of luminous efficiency attributable to localization ofcarriers may not be achieved. The atomic fraction of In of less than 20%or further less than 10% preferably reduces the influence of phaseseparation due to In.

On the other hand, when the intended emission wavelength is longer than470 nm, the atomic fraction of In is preferably from 1% to 50%, and morepreferably from 5% to 35%. If the atomic fraction of In is less than 1%,it is necessary to increase the atomic fraction of element X for thepurpose of bandgap adjustment, which may cause phase separation due toelement X. Thus, when the intended emission wavelength is greater than470 nm, the atomic fraction of In is set relatively high. However, inorder to minimize the influence of phase separation due to In, theatomic fraction of In is desirably less than 50%, and preferably lessthan 35%.

(Thickness of Light Emitting Layer)

The preferable thickness ranges of the well and barrier layers in thelight emitting layer according to this sixth embodiment are the same asin the first embodiment.

(Substrate for Growing Light Emitting Layer)

The substrate preferably used for formation of the light emitting deviceof the sixth embodiment is also the same as in the first embodiment.

(Number of Well Layers)

FIG. 15, similar to FIG. 9, shows the relation between the number ofwell layers included in the light emitting layer (multiple quantum wellstructure) of the sixth embodiment and the lasing threshold currentdensity. The light emitting layer in FIG. 15 includes anIn_(0.05)G_(0.95)N_(0.98)P_(0.02) well layer and a GaN_(0.99)P_(0.01)barrier layer (i.e., the atomic fractions of elements X, Y and In are2%, 1% and 5%, respectively). In the case of FIG. 15, similarly as inthe case of FIG. 9, continuous light-emission at room temperature waspossible when the number of well layers was at most 10. The well layersof at least 2 and at most 5 were preferable to further reduce the lasingthreshold current density. Although FIG. 15 relates to the lightemitting layer containing In_(0.05)Ga_(0.95)N_(0.98)P_(0.02) welllayer/GaN_(0.99)P_(0.01) barrier layer, the similar relation as in FIG.15 can be obtained with any light emitting layer satisfying therequirements of the sixth embodiment.

Now, explanation is given regarding the relation between the number ofwell layers included in the multiple quantum well light emitting diodeof the sixth embodiment and the luminous intensity, and regarding theinfluence of a substrate used for formation of that light emittingdiode. FIG. 16, similar to FIG. 10, shows the relation between thenumber of well layers included in the light emitting layer of the sixthembodiment and the luminous intensity. The light emitting layer in FIG.16 includes an In_(0.1)Ga_(0.9)N_(0.96)P_(0.04) well layer and aGaN_(0.995)P_(0.005) barrier layer. The luminous intensity in FIG. 16 isnormalized with the luminous intensity (broken line) of a light emittingdiode including a single quantum well layer ofIn_(0.1)Ga_(0.9)N_(0.96)P_(0.04).

When the conventional light emitting layer(In_(0.1)Ga_(0.9)N_(0.96)P_(0.04) well layer/GaN barrier layer) wasreplaced with the light emitting layer (number of the well layer wasvaried from 1 to 20) of the present embodiment and then the luminousintensity was measured, the maximum luminous intensity was about 1.6times the reference luminous intensity for normalization in FIG. 16.This means that the light emitting layer of the sixth embodiment issuperior to the conventional light emitting layer.

Further, as seen from FIG. 16 similar to FIG. 10, the number of welllayers ensuring high luminous intensity is preferably at most 10, andmore preferably at least 2 and at most 6, irrespective of the kind ofsubstrate. The similar results as in FIG. 16 can be obtained for asuper-luminescent diode device. Although FIG. 16 relates to the lightemitting layer including In_(0.1)Ga_(0.9)N_(0.96)P_(0.04) welllayer/GaN_(0.995)P_(0.005) barrier layer, any other light emitting layersatisfying the requirements of the sixth embodiment can obtain thesimilar relation of the number of well layers and the luminous intensityas shown in FIG. 16.

(Impurity in Light Emitting Layer)

The InGaNX well layer of the present sixth embodiment is a crystalcontaining element X, whose atomic fraction of In is low compared to aconventional InGaN well layer, and thus a local energy level due to Inis not likely to be generated. Thus, the influence of impurity such asSi added in the light emitting layer of the sixth embodiment on the PLluminous-intensity was similar to the case of the first embodiment.

(Light Emitting Layer and Emission Wavelength)

The emission wavelength of the nitride semiconductor light emittingdevice of the present sixth embodiment can be changed by adjusting theatomic fraction of element X in the InGaNX well layer. By way ofexample, Table 1 shows the relation between the emission wavelength andthe atomic fraction of As in the case that element X in the InGaNXcrystal is As. Further, Table 2 shows the relation between the emissionwavelength and the atomic fraction of P in the case that element X inthe InGaNX crystal is P. The atomic fractions of In listed in thesetables are calculated as In/(n+Ga) in the InGaNX crystal. Any emissionwavelength almost as intended can be obtained by forming the InGaNX welllayer with the atomic fraction of element X close to appropriate oneshown in Table 1 or 2.

TABLE 1 Atomic Fraction of In Emission Wavelength 1% 2% 5% 10% 20% 35%50% 380 nm 0.8 0.6 0.1 400 nm 2 1.8 1.3 0.4 410 nm 2.5 2.3 1.8 1 470 nm5.5 5.3 4.7 3.8 2.2 0.1 520 nm 7.5 7.3 6.7 5.8 4.1 1.9 0.1 650 nm 11.611.4 10.7 9.7 7.9 5.5 3.6 Atomic fraction (%) in InGaNX when element X =As

TABLE 2 Atomic Fraction of In Emission Wavelength 1% 2% 5% 10% 20% 35%50% 380 nm 0.5 0.4 0.1 400 nm 1.2 1.1 0.8 0.3 410 nm 1.6 1.5 1.1 0.6 470nm 3.4 3.3 2.9 2.4 1.4 0.1 520 nm 4.6 4.5 4.1 3.6 2.5 1.2 0.1 650 nm 76.9 6.5 5.9 4.8 3.4 2.3 Atomic fraction (%) in InGaNX when element X = P

In the conventional InGaN well layer, it was necessary to increase theatomic fraction of In to realize light emission of long wavelengths (of,e.g., green and red). By comparison, since the InGaNX well layercontains element X contributable to bandgap adjustment, the atomicfraction of In can be restricted to be low. This is preferable forsuppression of phase separation due to In and for improvement ofluminous efficiency.

(Light Emitting Layer and Half-Width of Emission Peak)

The half-width of emission peak in the case that the light emittinglayer of the sixth embodiment is applied to the light emitting diode ofthe fourth embodiment is similar to that in the case that the lightemitting layer of the first embodiment is applied to the light emittingdiode of the fourth embodiment. More specifically, in the vicinity ofthe wavelengths from 450 nm to 480 nm corresponding to blue lightemission, the half-width of emission peak was 65 nm in the conventionallight emitting layer containing InGaNAs well layer/GaN barrier layer,but it was 40 nm in the light emitting layer containing InGaNAs welllayer/GaNAs barrier layer.

(Method of Forming Nitride Semiconductor Laser Device)

As a method of forming the nitride semiconductor laser device of thepresent sixth embodiment, the method explained with reference to FIG. 1in the first embodiment is applicable without substantial changes. Inthis case, light emitting layer 106 may include, e.g., 4 nm thickIn_(0.05)Ga_(0.95)N_(0.98)P_(0.02) well layers and 8 nm thickGaN_(0.98)P_(0.02) barrier layers. As seen from FIG. 15, the number ofwell layers in the light emitting layer of at most 10 ensures lowthreshold current density and enables continuous light emission at roomtemperature. In particular, the well layers of at least 2 and at most 5advantageously decrease the threshold current density.

The light emitting layer of the present sixth embodiment can also beapplied, without substantial changes, to the light emitting layer of thenitride semiconductor laser device described with reference to FIG. 4,in which case the similar effects as in the laser device of FIG. 4 canbe obtained.

The light emitting layer of the sixth embodiment can also be applied,without substantial changes, to the light emitting layers of the nitridesemiconductor light emitting diode devices described with reference toFIGS. 6 and 7, providing the similar effects as in the light emittingdiode devices in FIGS. 6 and 7. In such cases, the light emitting layerof the device shown in FIG. 6 can include, e.g., 5-cycleIn_(0.05)Ga_(0.95)N_(0.97)P_(0.03) well layers (3 nm)/GaN_(0.99)P_(0.01)barrier layers (6 nm), and the light emitting layer of the device shownin FIG. 7 can include, e.g., 5-cycle In_(0.05)Ga_(0.95)N_(0.95)P_(0.05)well layers/GaN_(0.99)P_(0.01) barrier layers. Further, the lightemitting layer of the sixth embodiment can also be applied to a nitridesemiconductor super-luminescent diode device without substantialchanges. In this case, again, the similar effects as shown in FIG. 16can be obtained.

Seventh Embodiment

In the seventh embodiment, one of the nitride semiconductor lasers inthe above embodiments is applied to an optical apparatus. The GaNX orInGaNX well layer of the present invention includes at least one elementselected from As, P and Sb as element X. Inclusion of this element X inthe well layer can reduce the effective mass of electrons and holes inthe well layer, thereby increasing mobility of the electrons and holes.A smaller effective mass means that carrier population inversion forlasing can be obtained by introducing a smaller amount of current. Thesmaller effective mass also means that, even if electrons and holes inthe light emitting layer are lost due to recombination, electrons andholes can be newly introduced rapidly by diffusion. That is, it isconsidered that the nitride semiconductor laser device containingelement X is superior in self-sustained pulsation characteristics (ornoise characteristics) and has low threshold current density, comparedto a conventional InGaN base nitride semiconductor laser device of whichwell layer does not contain element X at all. However, such reduction ofthreshold current and improvement of luminous intensity has not beenachieved sufficiently by any conventional nitride semiconductor lightemitting device even including a well layer containing element X becausethe interfacial steepness between the well and barrier layers wasimpaired.

In the present invention, GaNY is used for the barrier layer in contactwith the GaNY or InGaNX well layer. This can improve the interfacialsteepness, and hence enables formation of a multiple quantum wellstructure of good quality. Then, it becomes possible to realizereduction of threshold current density and accompanying higher outputand longer life of the semiconductor laser, and to obtain asemiconductor laser excellent in noise characteristics. For example,when a nitride semiconductor laser of violet color (wavelengths of380-420 nm) is formed according to the present invention, it exhibits alow lasing threshold current density and strong resistance to noise,compared to a conventional InGaN based nitride semiconductor laser.Further, the laser device of the present invention can operate stably athigh output (50 mW) and at high temperature (60° C.), so that it issuitably applicable to a high-density recording/reproducing optical disk(shorter laser wavelength enables recording/reproduction of higherdensity).

FIG. 8 is a schematic diagram of the optical disk device including thenitride semiconductor laser device of the present invention. In FIG. 8,laser light is modulated by an optical modulator in accordance withinput information, and is collected on a disk via a lens. Thus, theinput information is recorded on the disk as pit arrangement. In thecase of reproduction, laser light optically influenced by the pitarrangement on the disk is detected by an optical detector via asplitter, and the detected light is converted to a reproduction signal.The recording and reproducing operations are controlled by a controlcircuit. In general, the laser output is on the order of 30 mW uponrecording and on the order of 5 mW upon reproduction. The nitridesemiconductor laser device of the present invention is also-applicableto a laser printer, a bar code reader, a projector including laserdevices of three primary colors (blue, green, red), and others.

Eighth Embodiment

In the eighth embodiment, the nitride semiconductor light emitting diodedevices according to the above embodiments are applied to a lightemitting apparatus (e.g., display apparatus or white light sourceapparatus).

The nitride semiconductor light emitting diode devices can be used for adisplay apparatus including at least one of optical three primary colors(red, green, blue). In the case of an amber light emitting diode deviceemploying a conventional InGaN based nitride semiconductor, the atomicfraction of In is high, causing considerable phase separation due to In.Therefore, such an amber light emitting diode did not reach the levelfor commercialization from the standpoints of reliability and luminousintensity. Element X included in the light emitting layer (especiallythe well layer) of the present invention serves to reduce the bandgapenergy of the well layer as does In. Thus, it is considered that afavorable light emitting layer can be obtained by including element Xinto the well layer, as it reduces the atomic fraction of In. However,in the nitride semiconductor light emitting device of the prior arthaving a well layer containing element X, interfacial steepness betweenthe well and barrier layers is liable to be impaired, making itdifficult to achieve the effect of fully improving luminous intensityand the like.

In the present invention, GaNY is employed for the barrier layer incontact with the GaNX or InGaNY well layer, which improves theinterfacial steepness between the well and barrier layers. Suchimprovement in turn enables formation of a multiple quantum wellstructure of good quality, and accordingly it becomes possible to form alight emitting diode or a super-luminescent diode that can emit light ofvarious wavelengths at high intensity with less color mottling.

The nitride semiconductor light emitting diode device according to thepresent invention can be used as one of light emitting diodes generatingoptical three primary colors, and also used in a white light sourceapparatus. Further, fluorescent coating may be applied to a nitridesemiconductor light emitting diode device of the present invention thatcan emit light in a range of ultraviolet region to purple region (on theorder of 360 nm to 420 nm), to form a white light source apparatus. Thiswhite light source apparatus can be utilized as a high-luminancebacklight consuming less power, in place of a halogen light sourceconventionally employed for a liquid crystal display. In other words, itcan be used as a backlight for a liquid display in a man-machineinterface of a portable notebook computer or a portable telephone,realizing a compact and high-definition liquid crystal display.

INDUSTRIAL APPLICABILITY

In a nitride semiconductor light emitting device having a light emittinglayer including a GaNX or InGaNX well layer and a GaNY barrier layer incontact with the well layer, the element X includes at least one elementselected from As, P and Sb, with the atomic fraction X/(N+X) of lessthan 20%, and the element Y includes at least one element selected fromAs, P and Sb. Accordingly, it becomes possible to obtain the nitridesemiconductor light emitting device improved in interfacial steepnessbetween the well and barrier layers and having a low threshold currentdensity or a strong luminous intensity.

1. A nitride semiconductor light emitting device formed on a substrate,comprising a light emitting layer including a quantum well layer and abarrier layer in contact with said well layer, wherein said quantum welllayer is formed of a nitride semiconductor containing Ga, N and anelement X said element X including at least one element selected fromAs, P and Sb, said well layer has an atomic fraction X/(N+X) of lessthan 30%, said barrier layer is formed of a nitride semiconductorcontaining Ga, N and an element Y, said element Y including at least oneelement selected from As, P and Sb, said barrier layer has an atomicfraction Y/(N+Y) of less than 15%, and said barrier layer has athickness of more than 1 nm and less than 40 nm, said substrate is apseudo GaN substrate including a GaN substrate layer grown to cover amain surface of a seed substrate and a plurality of stripe-shaped growthinhibiting films formed thereon, said growth inhibiting films serving torestrict growth of said GaN substrate layer, and wherein said lightemitting device includes a ridge stripe portion for narrowing current,said ridge stripe portion being formed avoiding a portion above thecenter in width of said stripe-shaped growth inhibiting film and aportion above the center in width of a region unprovided with saidgrowth inhibiting film.
 2. The nitride semiconductor light emittingdevice according to claim 1, wherein said element X and said element Yare the same element.
 3. The nitride semiconductor light emitting deviceaccording to claim 1, wherein a dose of said element Y is more than1×10¹⁶/cm³.
 4. The nitride semiconductor light emitting device accordingto claim 1, wherein said seed substrate is a nitride semiconductorsubstrate.
 5. The nitride semiconductor light emitting device accordingto claim 4, wherein said substrate has an etch pit density of less than7×10⁷/cm².
 6. The nitride semiconductor light emitting device accordingto claim 1, wherein said well layer has a thickness of more than 0.4 nmand less than 20 nm.
 7. The nitride semiconductor light emitting deviceaccording to claim 1, wherein said light emitting layer contains atleast one impurity selected from Si, O, S, C, Ge, Zn, Cd and Mg, and atotal dose of the impurity is more than 1×10¹⁶/cm³ and less than1×10²⁰/cm³.
 8. The nitride semiconductor light emitting device accordingto claim 1, wherein number of said well layers is at most
 10. 9. Thenitride semiconductor light emitting device according to claim 1,wherein said well layer further includes In, and said atomic fractionX/(N+X) is less than 20%.
 10. The nitride semiconductor light emittingdevice according to claim 9, wherein said element X and said element Yare the same element.
 11. The nitride semiconductor light emittingdevice according to claim 10, wherein an atomic fraction Y/(N+Y) isequal to the atomic fraction X/(N+X).
 12. The nitride semiconductorlight emitting device according to claim 9, wherein an atomic fractionY/(N+Y) is more than 2×10⁻⁵%.
 13. The nitride semiconductor lightemitting device according to claim 9, wherein said substrate is anitride semiconductor substrate or a pseudo GaN substrate.
 14. Thenitride semiconductor light emitting device according to claim 13,wherein said substrate has an etch pit density of less than 7×10⁷/cm².15. The nitride semiconductor light emitting device according to claim9, wherein said well layer has a thickness of more than 0.4 nm and lessthan 20 nm.
 16. The nitride semiconductor light emitting deviceaccording to claim 9, wherein said light emitting layer contains atleast one impurity selected from Si, O, S, C, Ge, Zn, Cd and Mg, and atotal dose of said impurity is more than 1×10¹⁶cm³ and less than1×10²⁰/cm³.
 17. The nitride semiconductor light emitting deviceaccording to claim 9, wherein number of said well layers is at least 2and at most
 10. 18. An optical pickup apparatus including the nitridesemiconductor light emitting device of claim 1, comprising a nitridesemiconductor laser device having a laser wavelength of more than 380 nmand less than 420 nm.
 19. A white light source apparatus including thenitride semiconductor light emitting device of claim 1, comprising alight emitting diode device or a super-luminescent diode device havingan emission wavelength of more than 380 nm and less than 420 nm.
 20. Adisplay apparatus including the nitride semiconductor light emittingdevice of claim 1, comprising a light emitting diode device having anemission wavelength of more than 450 nm and less than 480 nm and ahalf-width of emission peak of less than 40 nm.
 21. A method of formingthe nitride semiconductor light emitting device of claim 1, wherein saidwell layer and said barrier layer are grown by metallorganic chemicalvapor deposition, and a growth break interval of at least one second andat most 180 seconds is provided between growth of said well layer andgrowth of said barrier layer.
 22. A nitride semiconductor light emittingdevice formed on a substrate, comprising a light emitting layerincluding a quantum well layer and a barrier layer in contact with saidwell layer, wherein said quantum well layer is formed of a nitridesemiconductor containing Ga, N and an element X, said element Xincluding at least one element selected from As, P and Sb, said welllayer has an atomic faction X/(N+X) of less than 30%, said barrier layeris formed of a nitride semiconductor containing Ga, N and an element Y,said element Y including at least one element selected from As, P andSb, said barrier layer has an atomic fraction Y/(N+Y) of less than 15%,and said barrier layer has a thickness of more than 1 nm and less than40 nm, wherein said substrate is a nitride semiconductor substrate or apseudo GaN substrate and said substrate has an etch pit density of lessthan 7×10⁷/cm².
 23. An optical pickup apparatus including the nitridesemiconductor light emitting device of claim 22, comprising a nitridesemiconductor laser device having a laser wavelength of more than 380 nmand less than 420 nm.
 24. A white light source apparatus including thenitride semiconductor light emitting device of claim 22, comprising alight emitting diode device or a super-luminescent diode device havingan emission wavelength of more than 380 nm and less than 420 nm.
 25. Adisplay apparatus including the nitride semiconductor light emittingdevice of claim 22, comprising a light emitting diode device having anemission wavelength of more than 450 nm and less than 480 nm and ahalf-width of emission peak of less than 40 nm.
 26. A nitridesemiconductor light emitting device formed on a substrate, comprising alight emitting layer including a quantum well layer and a barrier layerin contact with said well layer, wherein said quantum well layer isformed of a nitride semiconductor containing Ga, N and an element X,said element X including at least one element selected from As, P andSb, said well layer has an atomic fraction X/(N+X) of less than 30%,said barrier layer is formed of a nitride semiconductor containing Ga, Nand an element Y, said element Y including at least one element selectedfrom As, P and Sb, said barrier layer has an atomic fraction Y/(N+Y) ofless than 15%, and said barrier layer has a thickness of more than 1 nmand less than 40 nm, said substrate is a pseudo GaN substrate includinga GaN substrate layer grown to cover a plurality of stripe-shapedgrooves and hills formed on a main surface of a seed substrate, andwherein said light emitting device includes a ridge stripe portion fornarrowing current, said ridge stripe portion being formed avoiding aportion above the center in width of said stripe-shaped groove and aportion above the center in width of said stripe-shaped hill.
 27. Anoptical pickup apparatus including the nitride semiconductor lightemitting device of claim 26, comprising a nitride semiconductor laserdevice having a laser wavelength of more than 380 nm and less than 420nm.
 28. A white light source apparatus including the nitridesemiconductor light emitting device of claim 26, comprising a lightemitting diode device or a super-luminescent diode device having anemission wavelength of more than 380 nm and less than 420 nm.
 29. Adisplay apparatus including the nitride semiconductor light emittingdevice of claim 26, comprising a light emitting diode device having anemission wavelength of more than 450 nm and less than 480 nm and ahalf-width of emission peak of less than 40 nm.
 30. The nitridesemiconductor light emitting device according to claim 26, wherein saidwell layer further includes In, and said atomic fraction X/(N+X) is lessthan 20%.
 31. The nitride semiconductor light emitting device accordingto claim 22, wherein said well layer further includes In, and saidatomic fraction X/(N+X) is less than 20%.
 32. The nitride semiconductorlight emitting device according to claim 26, wherein said seed substrateis a nitride semiconductor.